Curbs on emissions of carbon dioxide and other substances have been strengthened against a background of growing movements for environmental protection, and in the automobile world there is now vigorous development of electric vehicles (EVs) and hybrid electric vehicles (HEVs) alongside vehicles using fossil fuels such as gasoline, diesel oil and natural gas. In addition, the soaring prices of fossil fuels in recent years has acted to spur on the development of EVs, HEVs and the like.
The batteries used for such EVs, HEVs and the like are generally nickel-hydrogen secondary batteries or lithium ion secondary batteries. What is now being required of such vehicles is not only environmental compatibility, but also basic performance as automobiles, in other words, elevated driving capabilities. Therefore it is necessary not simply to enlarge the battery capacity, but also to increase the battery output, which exerts large effects on an automobile's acceleration and hill-climbing performances. However, when discharge of high output is implemented, large current will flow in the battery, and as a result there will be large heat-up due to contact resistance between the substrates and the collectors of the electrode assembly. Thus, batteries for EVs and HEVs are required not only to be large-sized and large capacity, but also to afford large current. Accordingly, in order to prevent electricity loss inside the battery and thereby reduce heat emission, many improvements have been carried out with regard to lowering the internal resistance by preventing welding faults between the substrates and collectors of the electrode assembly.
There exist the methods of mechanical caulking, welding and the like for electrically joining the substrates and collectors of the electrode assembly. Welding, which is joining by fusion, is appropriate as the electrical collection method for batteries of which high output is required. Also, in order to effect low resistance, the material used for the electrode assembly of a lithium ion secondary battery is copper (copper alloy) or aluminum (aluminum alloy), which however have the characteristics of low electrical resistance and high thermal conductivity, so that extremely large amounts of energy are required in order to weld them.
The following methods have long been known as methods for welding together the substrates and collectors of the electrode assembly:
(1) Laser welding (see JP-A-2001-160387)
(2) Ultrasonic welding (see JP-A-2007-053002)
(3) Resistance welding (see JP-A-2006-310254)
With the laser welding method, a high-energy laser beam is required because of a high reflectivity of about 90% of copper or copper alloy and of about 80% of aluminum or aluminum alloy with respect to yttrium-aluminum-garnet (YAG) laser light that is widely used to weld metals is high. There also exist the problems that when copper or copper alloy is laser-welded, the weldability varies greatly depending on the condition of the surfaces, and that the occurrence of spatter is unavoidable, as in laser welding of other materials.
With ultrasonic welding too, large amounts of energy are required because the thermal conductivity of the copper (copper alloy) or aluminum (aluminum alloy) welded material is high. Also, a negative electrode mixture may be dislodged by the ultrasonic vibration during welding. Accordingly, in the invention disclosed in JP-A-2007-053002, the electrode assembly, which is a generation element, is compressed during ultrasonic welding, so that dislodged negative electrode mixture will not enter inside the electrode assembly.
Further, with resistance welding, due to the copper (copper alloy) or aluminum (aluminum alloy) welded material having low electrical resistance and high thermal conductivity there exist the problems that large current needs to be input in a short time, that fusion bonding of the collectors and the bolt poles sometimes occurs during welding, and that melting or spark generation may occur at places other than the welds.
Thus, the three welding methods have their merits and drawbacks. In the interests of productivity and economy however, the resistance welding method, which has long been used as a method for welding between metals, will preferably be employed. But, especially in order to resistance-weld the collectors or collector receiving parts in a wound electrode assembly (see JP-A-2002-008708) of EV and HEV application prismatic batteries, which have exposed portions of positive electrode substrates at one end and of negative electrode substrates at the other, a great deal of welding energy is necessary in order to effect a firm weld, since the wound electrode assembly has a large number of stacked layers. Moreover, when the welding energy is rendered large for resistance welding, sometimes edge parts of the power collectors or collector receiving parts become molten or spark generation may occur at the edge parts, and an electrode bar for resistance welding may be fusion-bonded to the collectors or collector receiving parts.
Thus, when edge parts of the collectors or collector receiving parts become molten or a spark is generated at the edge parts, the parts are discolored and conductive metal particles generated by the molten metal or the spark may enter the inside of the electrode assembly to cause internal short-circuits. Further, since the edge parts of the collectors or collector receiving parts and a bottom side (a side to which an active material mixture is applied) of the substrates usually come close to each other, when the edge parts of the collectors or collector receiving parts become molten or a spark is generated at the edge parts, the bottom side (a side to which an active material mixture is applied) of the substrates may also be damaged. Further, when the electrode bar for resistance welding and the collectors or collector receiving parts are fusion-bonded to each other, a large amount of energy becomes required to separate the electrode bars and the collectors or collector receiving parts.